We use the Dri Eaz HEPA 500's. I replace the pre-filters and HEPA filters after every fire job and mold job. The filter may not be full, but can't get the smell out after a fire, and if you place on another job, it will stink the place up. We replace after mold for obvious reasons. And like Richard, I don't pay for them, my customers do. The HEPA filters for the Dri Eaz units are expensive around $150 each. We go through dozens per year. I hate it because the price you get for them in Xactimate barely covers the cost of the filter.